Monocarbide fiber-reinforced iron-base superalloy composite eutectic castings and method

ABSTRACT

A high performance eutectic casting comprised of an iron-base alloy matrix reinforced with 8 to 19 volume percent of uniformly aligned fibers of a metal monocarbide extending continuously entirely through the casting is made by directionally solidifying a melt containing the constituents of the monocarbide in amounts corresponding to the monocarbide content of the eutectic. The solidification rate is at least 1/4 inch/hour.

United States Patent [191 Tarshis et al.

[451 Feb. 19,1974

22 Filed: Mar. 1, 1972 21 Appl. No.: 230,928

[52] US. Cl 75/128 G, 75/129, 75/1305 [51] Int. Cl. C22c 39/20 [58] Field of Search 75/135, 128 G, 130.5, 129

[56] References Cited UNITED STATES PATENTS 3,528,808 9/1970 Lemkey et al. 75/170 l/l97l Thompson 148/32 3,554,817 3,564,940 2/1971 Thompson et a1. 75/134 3,671,223 6/1972 Thompson et a1. 75/122 Primary ExaminerL. Dewayne Rutledge Assistant Examiner-J5. L. Weise Attorney, Agent, or Firm-Gerhard K. Adam; Joseph T. Cohen; Jerome C. Squillaro 57 ABSTRACT A high performance eutectic casting comprised of an iron-base alloy matrix reinforced with 8 to 19 volume percent of uniformly aligned fibers of a metal monocarbide extending continuously entirely through the casting is made by directionally solidifying a melt containing the constituents of the monocarbide in amounts corresponding to the monocarbide content of the eutectic. The solidification rate is at least V; inch/- hour.

7 Claims, 2 Drawing Figures PAIENIED FEB 1 9 I974 SHEET 1 0F 2 PATENTED FEB 1 9 I914 SHEET 2 0F 2 MONOCARBIDE FIBER-REINFORCED IRON-BASE SUPERALLOY COMPOSITE EUTECTIC CASTINGS AND METHOD FIELD OF THE INVENTION BACKGROUND OF THE INVENTION The physical and chemical requirements of materials of construction'of gas turbine hot stage components have escalated since the advent of the'aircraft jet'engine because engine performance improvements re.- quire large operating temperature increases. For a long time, nickel-and cobalt-base alloys have served as the principal high-temperature materials of construction of jet engine buckets and vanes. The continual demand for materials of ever higher temperature. capabilities. has, however, resulted in the virtual exhaustion, of al.- loying possibilities of the nickel-base and cobalt-base- .systems and has led to efforts to develop composite ma terials. While dispersion-strengthening of nickelandcobalt-base alloys has not been promising, directional solidification of pseudobinary eutectics such as Ni Al Ni Cb has resulted in somewhat improved hightemperature mechanical properties (Thompson and. Lemke'y, Trans. ASMQ-VOI; 62, page 140, 1969). Simi-- lar properties have also been obtained inthe Nh-CbC" system (Lemkey and Thompson, Metal Trans, Vol. 2,. page 1537, June, 1971) in which the. typical direction ally solidified body has a relatively ductile matrix strengthened by an aligned fibrous phase. In fact, the former eutectic has been used in the production of gasturbine buckets, the respective phases in these eutectic products being aligned to maximize physicalpropertiesin the desired location.

Limitations imposed by the simple pseudobinary eutectic alloy upon the properties of these directionally solidified composites were not avoidable until it was re alized that the metal-monocarbide eutectics would'- withstand additions of alloying elements up to and somewhat beyond the point that the matrix qualities as a high temperature iron-base alloy, i.e., an alloy char acterized by deliberate additions of substantial amounts of nickel as anaustenite stabilizer, chromiumto improve elevated temperature environmental resistance and optional selectiveadditions of cobalt, aluminum, molybdenum, tungsten, niobium, tantalum, titanium and yttrium to provide precipitation hardening capability, improved matrix solid solution strength or both. In these complex matrix chemistry alloys, we have further found that reinforcing monocarbide fibers can be derived from tantalum, titanium, niobium,.zirconium, hafnium and vanadium, and'from combinations of two or more of these elements with carbon. We" provide carbon in these alloys in amount sufficient as a source of the metal monocarbide plus an amount in 2 equilibrium with the monocarbide in solution in the alloy matrix.

Additionally and very importantly, we have discovered that directionally solidified, monocarbide eutectic alloy castings which are free from: structural inhomoge--' ncity can be consistently produced. Thus, through criticalcontrol of the casting melt composition, one can in accordance with this invention make directionally solidified castings in which the fibrous monocarbide structure extends continuously from one end surface to another of an as-cast body. The necessity for trimming the casting, to eliminate non-fibrous portions can thereby be avoided without incurring any offsetting penalty of cost or product performance capability.

' Moreover, this result can be obtained in castings of a variety of shapes and'siz es with the continuous fibrous phase-forming capability being independent of casting dimensions andform.

SUMMARY OF THE INVENTION Thisinvention in both its method and article of manufacture aspects is predicated upon two basic novel concepts. First, we have found that engineering limitations such-as oxidation and hot-corrosion tendencies can be-avoided' while retaining the metal-monocarbide fiber-reinforcing effects through additions of alloying elements' to levels equivalent-to and possibly beyond those of conventional alloys. Secondly, we have found that the metal-monocarbide fibrous reinforcing phase can be formed so that it is coextensive with a directionally solidified casting'by providing a casting metal containingamountsof the monocarbide constituents corresponding to the monocarbide content of the eutectic.

thisinvention takes the form of an aligned composite cast eutectic body comprising an iron-base alloy matrix anda metal-monqcarbidefibrous phase within the matrix. providing reinforcement for the cast body. The fibrous phase consists essentially of aligned metalmonocarbide fibers extending continuously through the cast body fromone surface to the other of the body in its original as-cast condition.

DETAILED DESCRIPTION OF THE INVENTION The melt compositions provided according to this invention are iron-base alloys having a face-centered cubic crystal structure and containing at least 10.0 percent chromium and 5 to 14.0 percent of monocarbidefonning metals and 0.2 to 0.6 percent carbon. These compositions, as indicated above, may also contain substantial amounts of other metals as follows:

Cobalt trace to 20.0 percent Molybdenum trace to 5.0 percent Aluminum trace to 8.0 percent Titanium trace to 5.0 percent Yttrium trace to 1.0 percent Tungsten trace to 5.0 percent The volume fraction of metal monocarbide fiber in a composite eutectic casting of this invention depends 10.0 percent nickel, 14.0 percent tantalum, 0.6 percent carbon and 55.4 percent iron. The composition of the matrix of this alloy is close to that of the cast stainless steel alloy, CF-3.

upon the kinds and amounts of optional constituents of 5 For purposes of comparison, an article of pure iron the alloy such as aluminum. Additionally, the metalwas produced by directional solidification, and a third monocarbide content of the fibrous eutectic will be article was likewise formed from an iron melt containgoverned by the total composition, and particularly by ing tantalum and carbon equivalent to 10.0 tantalum the complexities of influences of constituents of the. carbide. alloy on the composition of the eutectic. For these rea- The COmPOSi Of these four 8110318 are Set Out in sons, it is necessary to determine in some manner Table I. rather precisely the composition of the eutectic and the amounts of the monocarbide constituents of the eutec- TABLE I tic so that the casting melt can be formulated.

One procedure which has been employed success- [5 Composition fully involves preparation of an iron-base alloy casting melt in which the carbon and monocarbide-forming Fe M Ta c metal contents are well into the hypereutectic range. pe-rac 9Q Q3 Directional solidification of this melt at a suitable rate e 100 such as 1 inch/hour results in an ingot or casting congig gig {8% 3:8 5' 3' taining in some increment of its length the metal monocarbide fibers. Then, chemical analyses of that segment of the casting will yield the eutectic composi- EXAMPLE I t1on and consequently the formulation of another casting melt of this analyzed eutectic composition to be The casting designated in Table I as FeCI'NiTaC was provided for directional solidification in accordance Produced y directional solidification Ofa melt Ofidenthe method of this invention to produce a new artitlcal composition at the substantially constant rate Of 7 1 l of hi invention, inch/hour in a temperature gradient of 250C per inch. The differences between the products of this inven- Practically no Segregation was found in the solidified tion and those resulting from the use of uch hypereuingot, and 1113 resulting mlCl'OStI'UCtUI'e had a matrix tectic casting melt are apparent from the drawings ac- Containing throughout substantially uniformly distribcompanying and forming a part of the specification, in and aligned monocarbide fibers, making "P P- hi h proximately 15.0 volume percent of the directionally FIG. 1 comprises a photograph of an as-cast ingot disolidified ingot. The fibers were essentially single crysrecti ll lidifi d f a h t ti l d tal monocarbides each containing essentially carbon three photomicrographs 150 dia.) of microstructures and tantalum The matrix corresponded in composition in three diff rent portions f one ingot as indi t d; to stainless steel containing chromium and nickel and d small amounts of tantalum and carbon essentially in FIG. 2 comprises a photograph array corresponding equilibrium with the" fibers. The tensile properties of to that of FIG. 1 in which the pictured ingot was pro- 40 this alloy, as well as those of pure iron and the other duced in accordance with the invention by directional two alloys of Table I, tested at a strain rate of 4 X solidification of a casting melt containing the eutectic l0 /min., are listed in Table II for room temperature amounts of the monocarbide constituents. and in Table III for elevated temperatures.

TABLE II Room Temperature Tensile Tests 0.2% Yield Ult. Tensile Alloy Stress (psi) Strength (psi) Elongation FeTaC 59,000 60,000 20.0 Fe 14,000 25,000 48.0 FeCrNiTaC 90,000 107,300 53.4 CIT-3* 37,000 77,000 55.4

' From reference work a v TABLE III Elevated Temperature Tensile Tests Test 0.2% Yield Ull. Tensile Alloy Temp. (F) Stress (psi) Strength (psi) 7c Elong.

FeTaC 1200 27,000 28,000 14 Fe 1200 2,000 4,000 72 FeCrNiTaC 1832 36,500 37,000 4.0 CF-3 1832 2,000

An article or product of the present invention is pro- The stress-rupture properties at elevated tempera ture of a typical article of this invention and of a typical stainless steel body are set out in Table o TABLE IV Elevated Temperature Stress-Rupture Properties As may be seen from the data of Tables II, III and lV, the metal monocarbide-reinforced iron-base eutectic alloys of the present invention has significantly superior high-temperature stress-rupture resistance and tensile 5 strength from room temperature to 1,832F, as compared to iron and stainless steel and tantalum carbidereinforced iron bodies prepared as described in the following examples:

EXAMPLE I] The casting designated inTable I as FeTaC was produced by directional solidification of a melt of the composition stated in Tablelin themanner described in Example I. In this directionally-solidified body, as in that of Example I, the monocarbides were in the form of aligned, uniformly dispersed TaC fibers throughout the iron-base alloy matrix, generally as illustrated in FIG. 2 These fibers made up about 18.7 volume percent of the directionally-solidified ingot.

EXAMPLE III A melt of pure iron was cast in the conventional manner (not directionally solidified) to produce an ingot having no second phase.

Herein and in the appended claims proportions or wise expressly specified.

It will be obvious to those skilled in the art upon reading the foregoing disclosure that many modifications and alterations in the specific compositions and microstructures disclosed as non-limiting examples may be made within the general context of the invention, and that numerous modifications, alterations and additions may be made thereto within the true spirit and scope of the invention as set forth in the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is: r g

1. The method of forming a high performance eutectic casting comprising an iron-base alloy matrix with 8 to 19 volume percent of a fibrous phase consisting essentially of aligned fibers of a metal monocarbide and extending continuously entirely through the casting in its ascast condition, which comprises the steps of preparing an iron-base alloy casting melt containing constituents of the monocarbide in amounts corresponding to the monocarbide content of the eutectic, and directionally solidifying said melt at a rate of at least V4 inch/hour.

2. The method of claim 1 in which the monocarbide is essentially TaC.

3. The method of claim 1 in which the casting melt consists essentially of about 55 percent iron, 20 percent chromium, 10 percent nickel, 14 percent tantalum and 0.6 percent carbon.

4. An aligned composite cast eutectic body comprising an iron-base alloy matrix and a metal-monocarbide percentages are stated on the weight basis unless otherfibrous phase within the matrix providing reinforcement for and imparting increased high-temperature stress-rupture life to the cast body, said fibrous phase consisting essentially of aligned metal-monocarbide fibers and extending continuously through the cast body from one surface to another of said body in its original cast condition.

5. A cast body as described in claim 4 in which the body is elongated and the fibrous phase extends longitudinally of the body from one end face to the other of the casting.

6. The cast body of claim 4 which is a jet engine turbine bucket.

7. The composite cast body of claim 4 of composition within the following percent ranges:

chromium 10.0 25.0

cobalt trace 20.0

aluminum trace 8.0

titanium trace 5.0

carbon 0.2 0.6

monocarbide 5.0 14.0 forming metal tungsten trace 5.0

molybdenum trace 5.0

ytt -92 T 0 8. H.

f $32333? I NITED STATES PATENT oFFIca 4 CERTIFICATE GF CO.ETI

q a a 3, 793, 008 Dated February 19, 1974 Patent No.

Invencm-( Lemuel A. Tars his and Edward R. Buchanari It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, c laim 7, line 47 "monocarbide 5.0 1'4. 0 forming metal" should read monqcarbide 5. 0 14. 0

' forrnlng metal v Signed" and sealed this 18th day of February 1975.

(SEAL) Attest C MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks 

2. The method of claim 1 in which the monocarbide is essentially TaC.
 3. The method of claim 1 in which the casting melt consists essentially of about 55 percent iron, 20 percent chromium, 10 percent nickel, 14 percent tantalum and 0.6 percent carbon.
 4. An aligned composite cast eutectic body comPrising an iron-base alloy matrix and a metal-monocarbide fibrous phase within the matrix providing reinforcement for and imparting increased high-temperature stress-rupture life to the cast body, said fibrous phase consisting essentially of aligned metal-monocarbide fibers and extending continuously through the cast body from one surface to another of said body in its original cast condition.
 5. A cast body as described in claim 4 in which the body is elongated and the fibrous phase extends longitudinally of the body from one end face to the other of the casting.
 6. The cast body of claim 4 which is a jet engine turbine bucket.
 7. The composite cast body of claim 4 of composition within the following percent ranges: chromium - 10.0 - 25.0 cobalt - trace - 20.0 aluminum - trace - 8.0 titanium - trace - 5.0 carbon - 0.2 - 0.6 monocarbide - 5.0 - 14.0 forming metal tungsten - trace - 5.0 molybdenum - trace - 5.0 yttrium - 0.2 - 0.8 